%documentstyle{article}                       % without NFSS
\documentclass{article}                       % with    NFSS
 \textheight15cm \textwidth11cm \parindent0mm \parskip 1ex plus2ex minus0.8ex
\begin{document}
default type                                                        \par \medskip
The following line acts differently with or without NFSS!           \par
{\bf These {\rm are \bf four} words.}                               \par\bigskip
\verb+\rm:+   {\rm abc ... xyz ABC ... XYZ  0123456789}             \par
\verb+\bf:+   {\bf abc ... xyz ABC ... XYZ  0123456789}             \par
\verb+\it:+   {\it abc ... xyz ABC ... XYZ  0123456789}             \par
\verb+\sl:+   {\sl abc ... xyz ABC ... XYZ  0123456789}             \par
\verb+\sf:+   {\sf abc ... xyz ABC ... XYZ  0123456789}             \par
\verb+\sc:+   {\sc abc ... xyz ABC ... XYZ  0123456789}             \par
\verb+\tt:+   {\tt abc ... xyz ABC ... XYZ  0123456789}             \par
\verb+\cal:+   {$\cal ABCDEFG ... XYZ$}                             \par
\verb+\mit:+   {$\mit aA1\Gamma$} \qquad (normal: $aA1\Gamma$)      \par
\verb+\boldmath:+ {\boldmath$\sqrt{3}$} \qquad (normal: $\sqrt{3}$) \par
\verb+\em:+   {word1 {\em word2 {\em word3} word4} word5}           \par
\end{document}
